Facts on Flash Floods in NSW

Flash floods occur when soil
absorption, runoff or drainage cannot adequately disperse intense
rainfall. The most frequent cause of flash flooding is slow-moving
thunderstorms. These systems can deposit extraordinary amounts of water
over a small area in a very short time.

The strong updrafts of air within
thunderstorms can suspend huge amounts of rain before releasing a
deluge onto the ground. Such rain can reach intensities of more than
100 mm per hour, provided the environment is humid enough to feed
sufficient moisture to the storm. Often topography acts to focus
thunderstorm development over a particular location, further
accentuating rainfall accumulation.

Flash floods in Wollongong on the
evening of 17 August 1998 caused extensive damage.

One such event occurred at Coffs
Harbour on the evening of 23 November 1996. Flash flooding from heavy
rainfall saw water rising to a height of around 1.5 metres in the
centre of the town and up to 300 people had to be moved to higher
ground. Tragically a woman drowned when she was swept away by the
rising flash flood. The insurance payout was around $30 million.

Flash floods are extremely
dangerous weather events as water in creeks, drains and natural
watercourses can rise very rapidly. On the evening of 26 January 1971
in Canberra, seven people died as flash flood waters from a nearby
thunderstorm flooded roadways near a drainage channel. It was estimated
that around 95mm of rain fell in one hour during this event.

In another event in Sydney on 7th
November 1984, 127 mm fell in one hour leading to damage of around $128
million (in July 1996 terms).